By default 3cx assumes the only purpose of the paging feature is to make emergency notifications and so they default to setting these to the highest priority rather than make it a configurable item in the phone provisioning.
You can make a custom phone template to change the behaviour to set the priority of multicast paging to be at the lowest priority to prevent an active call being interrupted. This would make your phone template unsupported so you would need to do this on your own. If your PBX is hosted by 3cx it does not allow modifications that would make it unsupported so the option to make custom templates will not be present in the hosted version.
If you decide to go with unsuported templates you will also see all your phones show up in the admin console in red to indicate they are using the custom and unsupported templates along with a warning on the main dashboard.
If you have problems you need to reprovision your phones to verify it works on the supported template before you can receive support. Also any time they update the templates you should compare what changes are being made to see if your custom template should be updated to match.
an example, I use many Yealink phones. I make this change to the built in templates:
#Enbale or diable the phone to handle the incoming multicast paging calls when there is a multicast paging call on the phone; 0-Disabled, 1-Enabled (default);
#If enabled, the phone will answer the incoming multicast paging call with a higher priority and ignore that with a lower priority.
multicast.receive_priority.enable = 0
#Configure the priority of multicast paging calls. It ranges from 0 to 10.
multicast.receive_priority.priority = 0
